1. Field of the Invention
The invention relates to an axial piston pump.
2. Description of the Related Art
An axial piston pump performs a pump action by receiving a fluid from a suction port into a piston chamber and discharging the fluid to a discharge port while relatively rotating a cylinder block with respect to a valving element. During this time, a fluctuation in pressure is caused in each of the piston chambers formed in the cylinder block. The fluctuation in the pressure acts as a vibromotive force on the pumping device and vibrates the pumping device. Consequently, noises are made. A process of the fluctuation in the pressure of one piston chamber includes a pressure rise process and a pressure drop process. If the pressure rapidly fluctuates in the pressure rise process and the pressure drop process, a pressure fluctuation curve includes many harmonic components. Consequently, the noises are particularly offensive to the ear.
There is an attempt to form a notch and a bypass port on a valving element in order to make the pressure fluctuation curve smooth in the pressure rise process and the pressure drop process (see Japanese Unexamined Patent Publication No. Sho 54-44208, for example). In an axial piston pump, a notch is formed continuously with respect to a discharge port, thereby making the pressure fluctuation curve in a piston chamber smooth in an early stage of the pressure rise process. The bypass port communicating with a suction port is formed on the valving element to make the pressure of the piston chamber escape to the suction port through the bypass port before the pressure of the piston chamber reaches that of the discharge port. Consequently, the pressure can be prevented from being rapidly raised in a late stage of the pressure rise process.
Moreover, a notch is formed continuously with respect to the suction port, thereby making the pressure fluctuation curve of the piston chamber smooth in an early stage of the pressure drop process. The bypass port communicating with the discharge port is formed in the valving element to lead the pressure of the discharge port to the piston chamber through the bypass port before the pressure of the piston chamber reaches that of the suction port. Thus, the pressure can be prevented from being rapidly dropped in a late stage of the pressure drop process.
As far as the pressure fluctuation curve of each of the piston chambers is concerned, it can be said that the above-mentioned structure can make the pressure fluctuation curve smoother. Pump noises, however, are made from all the pistons. Accordingly, even if the pressure fluctuation curve of each of the piston chambers is smooth, there are instances where the noises made by all the piston chambers include many harmonics.